The invention relates to a process for recovering components from a low boiler mixture which is obtained in the distillation of hydrogenation discharges from the preparation of polymethylols.
Polymethylols, for example neopentyl glycol (“NPG”) and trimethylolpropane (“TMP”), are used in the plastics sector for production of paint systems, coatings, polyurethanes and polyesters.
On the industrial scale, polymethylols are usually prepared by the Cannizzaro process. In order to prepare trimethylolpropane by this process, n-butyraldehyde is reacted with an excess of formaldehyde in the presence of an inorganic base. This likewise forms one equivalent of an inorganic formate as a coproduct. The separation of the salt of trimethylolpropane is complicated and requires additional work. Moreover, the inorganic salt—if it can be utilized in a profitable manner—must be worked up and purified. The occurrence of the coproduct otherwise constitutes a loss of the stoichiometrically used amounts of sodium hydroxide solution and formaldehyde. In addition, the yields in this inorganic Cannizzaro reaction are unsatisfactory in relation to n-butyraldehyde, since high-boiling constituents are formed in the course of the reaction, which cannot be utilized further.
Similar problems to those outlined for trimethylolpropane exist in the preparation of other polymethylols such as trimethylolethane (from n-propanal and formaldehyde) or trimethylolbutane (from n-pentanal and formaldehyde) or neopentyl glycol (from isobutyraldehyde and formaldehyde).
To avoid these disadvantages, WO 98/28253 disclosed a multistage process for preparing polymethylols, in which aldehydes having 2 to 24 carbon atoms are first condensed in a first stage (aldol reaction) with formaldehyde using tertiary amines as a catalyst to give the corresponding methylolalkanals, and then hydrogenated in a further stage (hydrogenation) to give the corresponding polymethylols. This multistage process is typically referred to as the hydrogenation process. This process is low in coproducts.
After the first stage of the hydrogenation process, unconverted aldehydes and a portion of the amine base are generally removed by distillation from the methylolalkanals formed and recycled.
In the distillation bottoms there remain—as well as the methylolalkanals formed—water, the adducts of formic acid and the tertiary amines used (amine formate) and formic acid itself.
In general, the polymethylolalkanal is obtained by these processes as a 20 to 70% by weight aqueous solution.
The polymethylolalkanal-containing solution is hydrogenated in a second stage in order to convert the polymethylolalkanals to the corresponding polymethylols, such as TMP or NPG.
The reaction discharge from the hydrogenation is typically an aqueous polymethylol mixture which comprises polymethylol, tertiary amine, water and organic secondary components, for example an adduct of tertiary amine and formic acid (amine formate). The aqueous polymethylol mixture is therefore typically purified by distillatively removing low boilers from the polymethylol compound.
In the distillative removal of the low boilers, the top product obtained in the condenser is a mixture of low boilers. For example, the low boiler mixture may comprise tertiary amine, unconverted aldehyde and water. More particularly, the low boiler mixture comprises alcohols which have formed by hydrogenation of the alkanals used in the process, such as isobutanol from isobutyraldehyde, or n-butanol from n-butyraldehyde, and methanol from formaldehyde.
The recycling of such a low boiler mixture into the first stage of the hydrogenation process (aldol reaction) is not advantageous, since the low boiler mixture comprises methanol and the corresponding alcohol from the unconverted aldehyde (isobutanol in the NPG process), which exert an adverse effect in the aldol reaction.
Relatively high methanol contents lead, for example, to by-products such as neopentyl glycol methyl ether and/or methyl acetals, through reaction of unconverted aldehydes, such as isobutyraldehyde and formaldehyde or methylolalkanes, with methanol.
Further by-products are, for example, 3,3-dimethoxy-2,2-dimethylpropanol, which is obtained in the trimethylolpropane or neopentyl glycol preparation.
The formation of the by-products mentioned leads to a reduction in the yield in relation to the aldehyde used as the reactant.
In the aldol reaction, methanol leads not just to a reduction in the yield as a result of side reactions, but it can additionally be removed from the remaining components only with very great difficulty, since methanol and the aldehydes used have a similar boiling point. When, however, the intention is to recycle the unconverted starting aldehydes into the aldol reaction, this means that a significant amount of the aldehydes must also be removed together with methanol, in order that aldehyde can be recycled into the process with a minimum of methanol content. Otherwise, there would be accumulation of methanol in the aldol reaction, which would result in the above-described increased formation of by-products.
Similarly, the presence of isobutanol in the aldol reaction leads to a worsened yield, since it is steam-volatile and can be removed by distillation from the aldehyde used as the reactant only with difficulty.
Utilization of the low boiler mixture by distillative separation is not trivial, since the boiling points of the substances present in the low boiler mixture are close to one another and some of these substances form azeotropes.
A disadvantage is especially the presence of isobutanol in the NPG preparation, since isobutanol forms, with water, a low-boiling heteroazeotrope with a miscibility gap.
In addition, the low boiler mixture contains dissolved carbon dioxide, which forms in the hydrogenation stage as a result of hydrogenation of formaldehyde, and formic acid, which is not present in free form but rather as salts with the amine in the aqueous solution. The presence of salts further complicates the distillation.
In the context of the invention, it has now been found that, even by means of a multistage distillation of the low boiler mixture from stage d), a separation into utilizable components is complicated by the fact that the unconverted aldehydes present in the low boiler mixture reduce the volatility of the tertiary amines used, especially of TMA, such that a removal of the tertiary amines from the other organic constituents succeeds only insufficiently or is associated with high yield losses.